Add metric start date tooltip to AI impact dashboard

What does this MR do and why?

For the AI impact dashboard, adds a tooltip to code suggestions usage rate metric values that pre-date when we started generating the metric (2024-04-04).

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

How to set up and validate locally

1. Ensure you have clickhouse configured
2. Enable the feature flags

# enables the UI table
Feature.enable(:ai_impact_analytics_dashboard) 

# enable clickhouse
Feature.enable(:clickhouse_data_collection)
Feature.enable(:event_sync_worker_for_click_house)

# enable the worker to sync code suggestions events to clickhouse
Feature.enable(:code_suggestion_events_in_click_house)

3. Seed data for the table FILTER=ai_usage_stats bundle exec rake db:seed_fu
4. Visit the groups analytics dashboard page: (ex. http://gdk.test:3000/groups/flightjs/-/analytics/dashboards)
5. Click the AI impact dashboard and validate that the metrics load as expected
6. Validate that the correct tooltip is shown for dates prior to April 2024

Related to #456108 (closed)